Alkali-developing type photosensitive resin compositions are widely used as solder resists for printed wiring boards. The solder resists aim at protecting circuits of printed wiring boards, and the tinting strength thereof is an important property that significantly affects the appearance of printed wiring boards and the concealing property of circuits.
Namely, when a solder resist has insufficient tinting strength, contamination and discoloring of a copper circuit formed on a printed wiring board become prominent, which decreases the appearance of the printed wiring board significantly. Furthermore, a step of mounting, which is a post step in the production of printed wiring boards, has been automated recently, and parts are attached by machines; however, a defect that a solder resist and a copper circuit cannot be recognized well during image recognition occurs. This phenomenon is also problematic in AOI (Automatic Optical Inspection) that is a final inspection of printed wiring boards.
On the other hand, as the photosensitive property of photosolder resists that are currently available in the market, general resists have a film thickness of around 20 μm and resolution property with a line width of from about 30 μm to 50 μm.
However, although conventional black solder resists are preferable in view of tinting strength (concealing property), they have problems that they have an insufficient curing depth and cannot form fine lines since a black coloring agent having absorbance from the ultraviolet region to the infrared region has been added thereto. Generally, a carbon black coloring agent that is excellent in weather resistance is used as a black coloring agent; however, since this coloring agent has large absorption in the ultraviolet region, when the content thereof is much, it absorbs ultraviolet ray and affects on transmitting property, and thus fine resolution cannot be obtained.
Thus, it was difficult to provide a black photosensitive resin composition that can form a solder resist layer that simultaneously satisfies fine concealing property and high resolution property.